The nuclear receptor superfamily is a group of ligand-inducible transcription factors that regulate specific target genes involved in biological processes as diverse as metabolism, development and reproduction (Mangelsdorf et al., “The Nuclear Receptor Superfamily: The Second Decade,” Cell 83(6):835-839 (1995)). Members in this family mediate the transcriptional response to hormones such as the sex steroids, adrenal steroids, vitamin D3, thyroid and retinoid hormones, in addition to a variety of metabolic ligands. Upon association with their cognate ligands, nuclear receptors acquire increased affinity for specific chromosomal binding sites, thereby recruiting a diverse group of coregulators to either activate or repress transcription initiation from the nearby promoter.
Nuclear receptors may be classified as one of two broad types based on their localization in the absence of ligand: Type I receptors, represented by estrogen receptor (ER), progesterone receptor (PR) and androgen receptor (AR), undergo nuclear translocation upon ligand activation and usually bind as homodimers to inverted repeating DNA half sites, called hormone response elements, or HREs. Type II receptors, such as thyroid hormone receptor and retinoic acid receptor, are often retained in the target cell nucleus regardless of the presence of ligand, and usually bind as heterodimers with RXR to direct repeats (Klinge et al., “Binding of Type II Nuclear Receptors and Estrogen Receptor to Full and Half-site Estrogen Response Elements in vitro,” Nucleic Acids Res 25(10):1903-1912 (1997)).
In addition to their normal functions, nuclear receptors have been implicated in many pathological processes, such as breast cancer, prostate cancer, ovarian cancer, diabetes and obesity (Novac and Heinzel, “Nuclear Receptors: Overview and Classification,” Curr Drug Targets Inflamm Allergy 3(4):335-346 (2004)). Some nuclear receptors have been important pharmacological targets for decades, considering the fact that over 13% of commonly prescribed drugs target nuclear receptors, with 15 of these drugs in the top 200 prescribed medicines (Via, “Nuclear Receptors: The Pipeline Outlook Report—Overview,” Cambridge Healthtech Institute (2010)). Among them, human estrogen receptor alpha (hERα) and progesterone receptor are most important biomarkers in breast cancer prognosis, in prediction of tumor response to hormone suppression therapy, and as drug targets in breast cancer treatments.
As members in the nuclear receptor superfamily share common structure architecture (Evans et al., “The Steroid and Thyroid Hormone Receptor Superfamily,” Science 240(4854):889-895 (1988); Mangelsdorf et al., “The Nuclear Receptor Superfamily: The Second Decade,” Cell 83(6):835-839 (1995)) and similar functionalities, hERα represents a model for Type I nuclear receptors. Furthermore, hERα is an important receptor to study given its significant role in breast cancer.
Besides skin cancer, breast cancer is the most commonly diagnosed cancer (about 28%) among U.S. women. In general, breast cancer treatments may include surgery to remove cancerous tissue, chemotherapeutic agents to kill cancer cells, radiation therapy to destroy cancerous tissue and other adjuvant therapies, depending on factors such as type and stage of the cancer, menopausal status, and prognostic/predictive biomarkers. A large number of breast cancers (up to 70%) are sensitive to the hormone estrogen, which functions as a mitogen to promote breast cancer cell proliferation. Such cancers express high level of ERα, and are called ERα-positive breast cancers. In contrast to tumor cells, ERα-positive cells in the normal human breast generally do not proliferate in response to estrogen. ERα in this subset of normal breast epithelial cells acts as the sensor of circulating or local estrogen concentrations, and the non-proliferating ER-positive cell is stimulated by estrogen to secrete growth factors for the paracrine control of neighboring ERα-negative epithelial cells proliferation (Clarke et al., “Dissociation Between Steroid Receptor Expression and Cell Proliferation in the Human Breast,” Cancer Res 57(22):4987-4991 (1997)).
Long term hormone blocking therapy is given to ERα-positive breast cancer patients. Estrogen functions are usually reduced in two ways. One way is to decrease estrogen synthesis using an aromatase inhibitor, represented by anastrozole and letrozole. Aromatase inhibitors block the final step in the conversion of androgen to estrogen. However, they are only suitable for post-menopausal patients (Buzdar, “New Generation Aromatase Inhibitors—from the Advanced to the Adjuvant Setting,” Breast Cancer Res Treat 75(Suppl 1): S13-17 (2002)). The other approach targets ER ligand binding pocket to competitively inhibit estrogen binding to ER using selective estrogen receptor modulators (SERMs), represented by tamoxifen (TAM). TAM is metabolized in the liver into active metabolites such as 4-hydroxy-tamoxifen (4-OHT), which antagonizes estrogen receptor activation in breast tissue, resulting in inhibition of tumor growth. However, it has been reported that TAM has estrogen-like activities in uterus, bone, liver, and the cardiovascular system, which enhances bone maintenance but increases the risk of endometrial cancer and causes alterations in liver function. Furthermore, after prolonged treatment, cancers gain resistance to the anti-estrogen treatment (Normanno et al., “Mechanisms of Endocrine Resistance and Novel Therapeutic Strategies in Breast Cancer,” Endocr Relat Cancer 12(4):721-747 (2005)), the mechanism of which remains poorly understood. This has led to the active pursuit of better ER modulators that display the optimal agonistic or antagonistic activities in various estrogen target tissues (Katzenellenbogen et al., Wouldiam L. McGuire Memorial Lecture “Antiestrogens: Mechanisms of Action and Resistance in Breast Cancer,” Breast Cancer Res Treat 44(1): 23-38 (1997)). Therefore, understanding ER actions, validating more drug target sites on the ER, and identifying new agents as drugs for these target sites are central goals for maximizing treatment opportunities in breast cancer therapy.
The present invention is directed to overcoming these and other deficiencies in the art.